upgrade: consider adding configlet upgrade command

[ee7]

Running configlet upgrade would make configlet upgrade itself. This command existed in the configlet for Exercism v2.

For my own interest, I had a local branch with a proof-of-concept implementation of this a while ago.

But the argument against it is:

• Maintenance burden: it adds lines of code and complexity to configlet.
• It increases the size of the configlet executable.
• We already have fetch-configlet.
• Even if we had configlet upgrade, we still want fetch-configlet for cases when configlet is not installed (e.g. in CI, or a user's first download), so we'd have maintain two codebases for downloading configlet.
• It's not like every command-line tool has self-upgrade functionality. In fact, it would be bad if every tool did - it's better handled by a package manager.
• It would have to handle the case of a user installing configlet system-wide, where configlet upgrade does not have write permission to upgrade itself. And I don't want to advise people to run configlet upgrade with e.g. sudo.

Related: #408, #153